In the past, the luminescent lamp lighting device used to light a single luminescent lamp by use of a single converter. Meanwhile, in the lighting device for lighting a number of luminescent lamps simultaneously, e.g. luminescent lamps as backlights for a liquid crystal panel, if increasing the number of luminescent lamps, the device would tend to be expensive with the necessity of providing a number of inverters.
For this reason, a luminescent lamp lighting device capable of lighting multiple luminescent lamps by a single inverter has been adopted recently. Nevertheless, the adoption comes a collision with the realities of differences in the lighting-up start voltage (referred to as “lighting voltage”) of the luminescent lamps. Due to load resistance characteristics of the luminescent lamps, additionally, the impedance after lighting is remarkably reduced in comparison with that before lighting.
When lighting a plurality of luminescent lamps by use of a single converter using a singe electrical transformer, only connecting of the luminescent lamps with the transformer would cause a luminescent lamp of low lighting voltage to be lighted ahead of the other luminescent lamps. Then, the impedance of the so-lighted luminescent lamp is reduced to cause the secondary voltage of the transformer to be lowered. Consequently, it becomes impossible to light the other luminescent lamps.
In this view, there has been adopted a method of individually lighting a plurality of luminescent lamps by an inverter using a plurality of electrical transformers. FIG. 1 shows the constitution of a conventional luminescent lamp lighting device for lighting two luminescent lamps by an inverter using two transformers. In FIG. 1, the shown luminescent lamp lighting device includes a direct-current power source 1, an inverter 2 using two transformers 24 and 26, a luminescent lamp 5 connected to the secondary side of the transformer 24 and a luminescent lamp 6 connected to the secondary side of the transformer 26.
The inverter 2 includes a switching element 21, a switching element 22 and a control circuit 23. The switching elements 21 and 22 are connected to the direct-current power source 1 in series. Respective gates of the switching elements 21 and 22 are connected to the control circuit 23. Connected in parallel to the switching element 22 are a parallel circuit having a primary winding of the transformer 24 and a primary winding of the transformer 25, and a condenser 25 also connected to the parallel circuit in series.
An end of the secondary winding of the transformer 24 is connected to an end of the luminescent lamp 5, while the other end of the winding is connected to the ground. An end of the secondary winding of the transformer 26 is connected to an end of the luminescent lamp 6, while the other end of the winding is connected to the ground. Respective other ends of the luminescent lamps 5 and 6 are connected to the ground.
The switching elements 21 and 22 are under the alternating ON/OFF control of the control circuit 23 to apply voltage of the direct-current power source 1 to the primary winding of the transformer 24, the primary winding of the transformer 26 and the condenser 25 intermittently. The voltage applied to the primary windings of the transformers 24 and 26 is respectively elevated by the transformers 24 and 26, so that high-frequency voltage occurs in the secondary winding of the transformer 24 and the secondary winding of the transformer 26. The high-frequency voltage occurring in the secondary winding of the transformer 24 is applied to the luminescent lamp 5, while the high-frequency voltage occurring in the secondary winding of the transformer 26 is applied to the luminescent lamp 6. Therefore, the conventional luminescent lamp lighting device of FIG. 1 is provided to light two luminescent lamps 5 and 6 by driving them with two transformers individually.
As another conventional luminescent lamp lighting device, there is well-known a device that a ballast element is connected to an output stage of an inverter using a single transformer so as to apply high-frequency voltage to a plurality of fluorescent lamps through the ballast element.
FIG. 2 shows the constitution of the other conventional luminescent lamp lighting device for lighting two luminescent lamps with the above structure where the ballast element is connected to the output stage of the inverter. In FIG. 2, the shown luminescent lamp lighting device has condensers C1 and C2 (as the ballast element) connected to the secondary winding of the transformer 24 to apply high-frequency voltage to the luminescent lamps 5 and 6 through the condensers C1 and C2.
In such a structure where the ballast element is connected to the secondary winding of the transformer 24, as the impedance of the ballast element is higher that the impedance of luminescent lamp, the impedance in the sight from the secondary winding would not vary so much even if the impedance of the luminescent lamp is lowered as a result of its lighting up. Thus, there is no reduction in the voltage occurred in the secondary winding of the transformer 24, allowing the other luminescent lamp to be lighted. Again, since the impedance of the ballast element is higher that the impedance of luminescent lamp, the difference of the composite impedance of the luminescent lamps and the ballast element gets smaller in spite of a difference between respective impedances of the luminescent lamps, allowing the luminescent lamps to be equalized in their luminance.
Additionally, Japanese Unexamined Patent Publication (heisei) No. 7-249494 discloses a luminescent lamp lighting device for lighting a plurality of luminescent lamps with a single transformer, in which a ballast element, the luminescent lamps and a two-way switch are connected in series on the secondary side of the transformer. This luminescent lamp lighting device is equivalent to the conventional luminescent lamp lighting device of FIG. 2 provided that the luminescent lamps and the ballast element are connected to a two-way switch in series, allowing the luminescent lamps to be lighted on and off under the ON/OFF control of the two-way switch.